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On a bicycle, the cassette or the freewheel (previously also known as a block or cluster) is the set of rear sprockets that attaches to a hub on the rear wheel. The cassette is a later development than the freewheel. Cassettes and freewheels are not the same, but because many bicycle users do not understand the differences, the terms are often used interchangeably, though incorrectly.[1]
[edit] Freewheels
A freewheel consists of the rear cogset and a ratcheting mechanism in a single replaceable assembly. Freewheels must be used with threaded hubs. The freewheel is attached to the hub by means of a right-hand thread. Traditional rear hubs had a standardized set of threads onto which a standard freewheel was screwed. This allowed many different brands of freewheels to be mounted on different brands of hubs. Unlike many cassettes, the individual sprockets in a freewheel can be removed or replaced if necessary. Most bicycles used this system of freewheels and threaded hubs until the late 1980s.[1] [edit] CassettesOver the last few years the cassette type hubs, called freehubs, have largely replaced the conventional threaded rear hub. A freehub incorporates the ratchet mechanism into the hub body. The ratchet mechanism is still replaceable on most hubs. Many users lament the loss of the brand interchangeability that existed with most freewheel systems. However, cassette systems have a major advantage in that the drive-side axle bearing can be out near the frame, rather than being back towards the center of the axle behind the freewheel. This greatly reduces the stress on the rear axle, so it does not fail as often and can be lighter. Cassettes work so well that they have become the new standard. Most quality bikes made since the late 1980s have used this greatly improved design. Cassettes are distinguished from freewheels in that a cassette typically has a series of straight splines that form the mechanical connection between the gears and the hub. The entire cassette is retained on the hub by means of a screwdown lockring. Some cassette systems from the late 1980s and early 1990s use a threaded small cog to hold on the larger splined cogs, the entire set referred to as a cluster. Cassettes resemble freewheels, but lack a contained freewheel mechanism. The sprockets are commonly sold as a set, called a "cassette". The sprockets in a cassette are usually held together by three small bolts or rivets for ease of installation. These bolts or rivets are by no means necessary, they just make it easier to keep the sprockets and spacers in the correct order and position when they are removed from the ratchet body. Individual sprockets are also available. When the sprockets need to be replaced or the user wishes to replace them to change gear ratios, only the sprockets are replaced, not the ratchet mechanism. 
[edit] Number and width of sprocketsOver time, the number of sprockets in a freewheel and cassette has increased, from three and four before WWII, to five and six from the 1950s to the 1970s, to seven to eight in the 1980s, then nine, ten and now eleven for racing bicycle groupsets. From seven to eight speed, the spacing between sprockets was decreased and the rear spacing of the frame slightly widened to accommodate the greater number. With nine speed, the sprockets were made thinner, and thinner still for ten speed. Because of the thinner sprockets, a thinner chain should also be used. The use of thinner metal parts in both these cases has had the effect of shortening the life-span of the chain and cogs due to "stretching" of the chain (frictional wear of the load-bearing parts of the chain causing elongation) and wear-and-tear. [edit] Improvements in shiftingBeginning with Shimano's Hyperglide system, and continuing with Campagnolo's UltraDrive, and SRAM's OpenGlide, groupset manufacturers have engineered cassettes with complex tooth profiling designed to pick up and drop the chain, which itself will be specifically manufactured to ease shifting. This, coupled with the adoption of combined brake and shift levers, allows for shifting under greater load than was previously possible, such as when out of the saddle. [edit] See also
[edit] References
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